lis AN AMERICAN TEXT- HOOK OF PHYSIOLOGY. 



is remembered thai the lungs and the heart with their great blood-vessels are 

 placed within an air-tight cavity, that the lungs become inflated through the 

 aspiratory action of the muscles of inspiration, and that during inspiration 

 intrathoracic negative pressure is increased, it is easy to understand how the 

 action which causes inflation of the lung- must affect in like manner such 

 hollow elastic structures as the heart and the great blood-vessels, and thus 

 influence the circulation. It is obvious, however, that this influence must make 

 itself felt to a more marked degree upon the vessels than upon the heart, and 

 upon the flaccid walls of the vein- than upon the comparatively rigid walls of 

 the arteries. Moreover, the effects upon the flow of blood through the vessels 

 entering and leaving the thoracic cavity must be different : the inflow through 

 the veins must he favored, and the outflow through the arteries hindered; but 

 it i- upon the flaccid veins chiefly that the mechanical influences of inspiration 

 are exerted. If the thoracic cavity be freely opened, movements of inspiration 

 no longer cause an expansion of the lungs, nor is there a tendency to distend 

 the heart and the large blood- vessels; if, however, in an intact animal the out- 

 let of the thorax be restricted, as by pressure upon the trachea, the force of the 

 inspiratory movement would make itself felt chiefly upon the heart and the 

 vessels, and it is under such circumstances that the maximal influences of in- 

 spiration upon the circulation are observed. The lungs on the one hand and 

 the heart and its large vessels on the other may be regarded as two sacs placed 

 within a closed expansible cavity, the former having an outlet communicating 

 with the external air, and the latter having inlets and outlets communicating 

 with the extrathoracic blood-vessels, both being dilated when the thorax ex- 

 pands and constricted when it contracts. Moreover, the blood-vessels in the 

 lungs may be compared to a system of delicate tubes placed within a closed 

 distensible bag and communicating with tubes outside of the bag, simulating 

 the communication of the venae cavae and the aorta with the extrathoracic 

 vessels. When such a bag is distended the tubes undergo elongation and 

 narrowing, and their capacity is increased. The narrowed vessels also tend 

 to be expanded, owing to the negative pressure present ; and thus have their 

 capacity further increased. The lungs in the same way, when expanded by 

 the act of inspiration, exhibit a simultaneous elongation and narrowing of the 

 intrapulnionary vessels, which results, however, in an increase in their total 

 capacity. 



I)urin<_ r expiration negative intrathoracic pressure becomes less, so that 

 there is a gradual return of the elongated and narrowed intrathoracic vessels 

 to that condition which existed at the beginning of inspiration ; at the same 

 time the intrapulnionary vessels are not only subjected to the passive influ- 

 ence of the declining intrathoracic pressure, but are actively squeezed, as it 

 were, between the air in the lungs on one side and the expiratory forces 

 expelling the air on the other. Thus we have during expiration passive and 

 active agents combining to bring about changes in the capacity of the intra- 

 pulnionary vessels. 



The mechanical effects of the movements of respiration upon blood-press- 



